1. Field of the Invention
This invention relates to a distance measuring device, particularly a level measuring device, as well as to a frequency converter circuit for use in said radar distance measuring device.
2. Description of the Prior Art
For non-contacting distance measurement by means of microwaves, particularly pulsed radar and frequency modulated continuous wave radar (FMCW radar) are known. In pulsed radar short microwave transmission pulses are emitted periodically which are reflected by the object being measured and re-received as an echo signal after a transit time depending on the distance involved. In pulsed radar the location in time of the echo signal within each transmission/reception period directly corresponds to the distance of the object being measured. In FMCW radar a continuous microwave is emitted which is periodically linearly frequency modulated, for example, according to a saw-tooth function. The frequency of each received echo signal thus exhibits with respect to the instantaneous frequency which the transmission signal has at the time of reception, a difference in frequency which depends on the transit time of the echo signal. The difference between the frequency of the transmission signal and that of the reception signal which may be obtained by mixing both signals and evaluating the Fourier spectrum of the mixed signal, thus corresponds to the distance of the reflecting surface of the object from the duplexer antenna.
In radar distance measurement the intention is to permit resolving several objects arranged at short distances one behind the other in the propagation path of the microwaves and having more or less the same degree of reflection, to be able to distinguish the object to be measured, particularly the surface of the material in the container, from structures incorporated in the container and the like. For a specified distance resolution of 15 cm the length of the emitted pulses in a pulsed radar arrangement must not exceed a maximum of 1 ns. For the same distance resolution when using the FMCW method the frequency of the transmission signal must permit changing by at least 1 GHz, e.g. from 4.8 GHz to 5.8 GHz. Higher transmission frequencies of, for instance 24 GHz are of advantage in that due to the improved focussing measurements are possible in small or slim containers, a higher gain is achieved with smaller antennas, and high-frequency lead-through on containers having a high internal pressure is simplified. However, employing elevated transmission frequencies is made less attractive by the increased expense thereof. Furthermore, applications exist in which the use of a lower transmission frequency is more favorable, for example in the case of liquid surfaces which tend to foam. There is thus a need for radar distance measuring devices which are capable of operating at an elevated transmission frequency; at the same time there is a continuing requirement for radar distance measuring devices which operate with a relatively low transmission frequency of, for instance, 5.8 GHz.